The invention is based on a measuring element for a device for determining the mass, or flow rate, of a flowing medium, in particular the aspirated air in internal combustion engines.
In such devices, also known as hot film air flow rate meters, the resistor paths of a temperature sensor and a compensation resistor on the one hand and the resistor path of the measuring resistor on the other, together with two compensation resistors, form a Wheatstone bridge, the bridge diagonal voltage of which is applied to a servo amplifier. The output voltage of the servo amplifier serves as a heating voltage for the heating resistor of the measuring element. The resistor tracks for the measuring and heating resistors of the measuring element are disposed on the substrate in such a way that a good thermal contact exists between them. As a result, the measuring resistor is heated to an excess temperature that is far above the temperature of the medium. If the flow quantity flowing across the measuring element changes, then the Wheatstone bridge is detuned, because the altered convective heat transfer changes the temperature of the measuring resistor as well, and because the measuring resistor has a temperature coefficient other than zero. The servo amplifier thereupon changes the output current for the heating resistor. Via the closed control loop, changes in the measuring resistor due to an outgoing or incoming quantity of heat are compensated for by varying the heating output of the heating resistor. The heating current or the output voltage of the servo amplifier is thus a measure of the flow rate of the flowing medium. Temperature fluctuations in the flowing medium are compensated for by the series connection of the temperature sensor and the compensation resistor.
In a known measuring element (German Patent Application 36 38 138 A1; U.S. Pat. No. 4,777,820) of the type referred to at the outset, for the device described, the distribution of the resistor tracks on the substrate is such that the resistor tracks are lined up with one another, parallel to one another in the flow direction. The resistor track for the compensation resistor is disposed between the resistor track for the temperature sensor and the resistor track for the measuring resistor, on the same side of the substrate, and the resistor track for the heating resistor is disposed on the other side of the substrate, directly opposite the resistor track for the measuring resistor. The various resistor tracks are separated from one another by slits extending transversely to the flow direction in the substrate; this brings about a temperature decoupling between the resistor tracks. As a result of these separating slits, the substrate has three tongues of equal length, of which the first two tongues in the flow direction each have one resistor track, while the last tongue in the flow direction has the resistor tracks for the measuring resistor and the heating resistor.
It has been found that despite careful manufacture and optimized adhesive bonding, spreading apart of the tongues cannot be precluded, and deviations of the tongues from the optimal alignment are relatively frequent and are of variable magnitude. As the flowing medium flows around the measuring element, this spreading of the tongues has a major influence on the form of the boundary layer and leads to increased amounts of rejects for the sake of the required characteristic curve of the measuring element.